1. Field of Invention
The present invention relates to a robot arm structure, and more particularly to a wrist structure of a robot arm.
2. Related Art
The robot or robot arm has been widely applied in various different fields such as production and manufacturing, warehouse management, and intelligent nursing. The robot or robot arm is usually formed by a structure with a plurality of support arms connected together. The pivoting motion of each support arm is controlled by driving joints connecting the support arms.
Generally, a joint-type industrial robot has six rotation shafts, among which four rotation shafts serve as the rotation shafts of the robot arm for achieving the wide-angle movement in a large range, and the other two rotation shafts are wrists of the robot arm for achieving positioning and orienting functions such as gripping and extracting. For example, U.S. Pat. No. 5,178,031 has disclosed a wrist for an industrial robot, in which the wrist part is only supported by a single side, and the rotation movement of the wrist may be influenced if the load is excessively heavy, and furthermore, the rotating motor at the wrist is exposed outside and cannot be operated in a dangerous environment (such as an oil gas or an aqueous gas).
In addition, for example, U.S. Pat. No. 7,028,578 has disclosed a wrist driving mechanism for a robot, in which three rotation shafts respectively drive a gear train by a drive motor and then the gear train further drives a speed reducer for performing transmission. However, in U.S. Pat. No. 7,028,578, a plurality of bearings and a plurality of gear elements are taken as a transmission interface. As a result, the assembling process is difficult and the cost is high.
Furthermore, for example, U.S. Pat. No. 5,231,889 has disclosed a wrist unit for an industrial robot, in which the wrist is located between two side holes of an arm, two bearings are inserted into through holes on both sides, the outer side of the arm is fixed through suppress caps, and the rotation shaft of the motor on the right side is connected to the rotation shaft of the wrist through a belt. In U.S. Pat. No. 5,231,889, the motor on the right side is directly fixed to the inner side of the arm, and the rotation shaft of the wrist is fixedly soldered to the wrist, so that the relative distance between the motor on the right side and the rotation shaft of the wrist is constant. However, the motor on the right side drives the rotation shaft of the wrist through a belt. The belt has its service life, and when the belt is in fatigue, the motions of the wrist cannot be positioned any more. Thus, such inaccurate positioning problem cannot be solved unless a new belt is used to replace the old one. Furthermore, all common robot arms need to be provided with a speed reducer, and the speed reducer reduces the revolution number output by the motor to a desired revolution number, so as to obtain a relative large torque. In U.S. Pat. No. 5,231,889, the motor on the right side needs to be configured with a speed reducer. However, the rotation shaft of the wrist is directly fixed to the wrist by soldering, and then the bearings are taken as an interface for reducing frictions. That is to say, in the design of the wrist unit disclosed in U.S. Pat. No. 5,231,889, no suitable location is provided for mounting the speed reducer. If an additional mechanism is further provided for mounting the speed reducer, the robot arm definitely has a more complicated structure, and necessary parts and components are further increased, thereby resulting in the increased cost. Furthermore, if an additional mechanism is further provided for transmitting the output of the speed reducer to the wrist, the wrist may fail to obtain the predetermined torque due to the transmission loss caused by the additional mechanism, and as a result, the motions of the wrist cannot be performed accurately.